1. Field of the Invention
The present invention relates to a vibration measurement apparatus and a recording medium containing a vibration measurement program and in particular, to a vibration measurement apparatus of non contact type calculating a displacement of an object utilizing the Doppler effect of the laser beam and a recording medium containing such a vibration measurement program.
This vibration measurement apparatus can be applied to various experiment analyses of the automobile production technology or the like. More specifically, engine vibration analysis, body vibration, analysis of noise within an automobile, muffler vibration analysis, and the like. Since the vibration measurement apparatus is non-contact type and capable of accurately measuring vibration in a very small area, it can be applied to inspection in semiconductor production as well as inspection of a drill and other instruments. Furthermore, the vibration measurement apparatus can be used for detecting can be applied for detecting abnormal vibration in a production plant using a motor as well as leak from the water pipe and gas pipe. Furthermore, the vibration measurement apparatus can be employed for checking sweetness of watermelon other big fruits by knocking on the fruits to generate a sound. Here, an "object to be measured" includes a vast range from engines to watermelons.
2. Description of the Related Art
Conventionally, a speed indicator utilizing the laser beam Doppler effect has been used. The laser beam Doppler effect can also be used for measuring a displacement of an object to be measured as follows. That is, an oscillated laser beam is mixed with a reflected beam in a laser resonator. As a result, a beat wave of the laser beam can be detected by a photo detector (photodiode and the like). Here, if the object has moved a certain distance (half of the laser beam wavelength .lambda.), a chopping wave appears. Accordingly, by counting the number of the chopping waves, it is possible to calculate the displacement of the object.
However, a displacement smaller than the certain distance of one chopping wave cannot be accurately measured. Especially, when the object is vibrating, it is difficult to detect a turning point. That is, displacing object moves to the maximum point by vibration and then displacement becomes zero. After this, the object starts to displace in the opposite direction. Here, the chopping wave has a gentle slope and it becomes difficult to detect the turning point, i.e., the moment when the vibration direction has changed.
To cope with this, there is a method to calculate the turning point according to the highest speed position in on vibration cycle. The highest speed position can be detected in a comparatively stable state. However, if the object vibrates with a plurality of frequency components, a displacement in one direction is not symmetrical to a displacement in the other direction. This disables to calculate a turning point according to the highest speed position in one vibration cycle.
The vibration measurement apparatus utilizing the Doppler effect of the laser beam intends to perform in non-contact state the function of the acceleration pickup to be fixed on the object to be measured. Accordingly, it is desired to preferably measure the vibration characteristics of the object to be measured. However, If the turning point cannot be calculated or detected accurately, a displacement amount is added without turning. Thus, it is impossible to calculate with a high accuracy.
Moreover, when the vibrating plane contains a displacement of large highest speed and a small highest speed, a plurality of waves are overlapped and it is impossible to calculate a turning point for the small highest speed. Furthermore, it is difficult to identify a turning point of vibration which has once slowed down speed and then increases the speed in the same direction.